Polypropylene lining

ABSTRACT

A lining for a metal, a lining powder and a lining resin composition which comprise a mixture of a maleic acid-modified polypropylene and a polypropylene, or a mixture of the above-mentioned mixture and an ethylene propylene rubber and/or an antioxidant; and a lining method.

This is a divisional application of application Ser. No. 08/527,092, filed Sep. 12, 1995, now abandoned, which is a division of application Ser. No. 08/285,998, filed Aug. 4, 1994, now abandoned, which is a division of application Ser. No. 08/184,290, filed Jan. 21, 1994, now abandoned.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a heated/melted polypropylene lining, a lining powder, a polypropylene resin composition for lining and a method for preparing the polypropylene lining, which are applicable to metal (carbon steel) materials for use in an exhaust gas desulfurization device, a gas purification plant, a seawater-fresh water converter, a thermal power plant and the like.

Heretofore, as the polypropylene lining, there is known the so-called sheet lining in which the surface of a sheet or a pipe of the polypropylene is treated by corona discharge or with a mixed solution of concentrated sulfuric acid and chromic acid to impart adhesive properties to the surface, coated with an adhesive such as an epoxy resin, and then stuck on a material to be lined. However, the so-called heated/melted polypropylene lining has not been suggested at all in which a material to be lined is heated up to a temperature more than the melting point of the polypropylene, and the heated surface of the material is then sprinkled with a polypropylene powder to form a film thereon, as in the case of the lining of a polyethylene and a fluororesin. In addition, there has been neither marketed nor suggested a lining powder for heated/melted polypropylene lining in which a material to be lined is sprinkled with the powder, and this powder is then heated/melted.

Furthermore, there have not been suggested a polypropylene sheet lining in which a material to be lined is heated up to a temperature more than the melting point of a polypropylene, a sheet or a stick of the polypropylene having adhesive properties is then fused to the heated surface of the material to form a lining and to thereby secure the adhesion of the polypropylene sheet or stick to the metal.

In addition, another lining method has not been suggested at all in which a metal surface is sprinkled with a polypropylene powder, and the powder is then heated/melted, and then cooled/solidified.

The above-mentioned polypropylene lining, in which the surface of a sheet or a pipe is treated, coated with an adhesive, and then used for lining, has the following problems, and so it is scarcely applied on an industrial scale:

(1) Since the adhesive is used for the lining, a defect such as an adhesive failure tends to occur.

(2) The polypropylene sheet or pipe is pressed against a material to be lined by worker's hands or by the use of a suitable tool to achieve the adhesion, and therefore it is difficult to apply the polypropylene lining to a material having a complicate shape such as a bent portion of a pipe or a T-shaped pipe, or a material having a small diameter.

(3) When metal pipes are lined, polypropylene pipes which have been previously extruded by an extruder are used. Therefore, in a plant in which the metal pipes having various diameters are used, the polypropylene pipes which are fit for the diameters of these metal pipes are required. Thus, for the manufacture of these polypropylene pipes having the various diameters, many molds are necessary, which increases the cost of the polypropylene pipes.

In general, the polypropylene has been used for injection-molded articles, extruded articles, films and the like, but the commercially available polypropylene can adhere to the metal surface while it is in a heated/melted state, but it peels therefrom inconveniently when it is cooled/solidified. Therefore, as described above, one of the polypropylene lining techniques has been only put to practical use, and this practical polypropylene lining comprises treating the surface of the extruded sheet or pipe to modify the surface, i.e., to impart adhesive properties to the surface, and then lining a metal material with the sheet or the pipe by the use of an adhesive.

As described above, the commercially available polypropylene cannot be stuck on the metal material by a heating/melting method, because the solidification shrinkage ratio of the polypropylene is large; creep stress caused by the case where the solidification shrinkage remains for a long period of time even at a relatively low temperature (ordinary temperature) is much larger than when a polyethylene or a fluororesin is used; and there is the influence of an additive such as an aggregate which is added to the polypropylene in order to improve mold release properties in the step of the injection molding or the extrusion, but detail factors are indefinite.

OBJECT AND SUMMARY OF THE INVENTION

In view of the above-mentioned technical level, the present invention has been completed, and an object of the present invention is to provide a heated/melted polypropylene lining material and its lining powder which can maintain excellent chemical resistance, heat resistance and mechanical properties of the polypropylene and which has excellent adhesive properties.

Another object of the present invention is to provide a lining material and its lining powder for metals which can maintain excellent chemical resistance, heat resistance and mechanical properties of the polypropylene and which can inhibit the deterioration of the resin.

Still another object of the present invention is to provide a polypropylene resin composition for lining which can maintain excellent chemical resistance, heat resistance and mechanical properties of the polypropylene and which has excellent adhesive properties, and a method for preparing this composition.

A further object of the present invention is to provide a method for extremely simply lining a metal (carbon steel) with a polypropylene.

The present inventors have intensively conducted experiments and investigations with the intention of developing a heated/melted polypropylene lining technique using a powder which can be applied to metal materials or similar materials having intricate shapes as well as large and small diameters, and as a result, they have found that after a heating/melting step, the polypropylene, when used singly, peels at the time of cooling/solidification, but when the polypropylene is mixed with a maleic acid-modified polypropylene in a suitable ratio, the resin does not peel to obtain a good lining film having strong adhesive force.

That is, the inventions are directed to:

(1) a polypropylene lining characterized by comprising a heated/melted mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/10,

(2) a polypropylene lining powder characterized by comprising a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 500 μm or less and a polypropylene powder in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=10/1 to 1/10, and

(3) a polypropylene lining powder obtained by melting and kneading a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/10, and then milling the mixture so that an average particle diameter may be 500 μm or less.

A metal which is useful in applying the present inventions is usually carbon steel. Furthermore, the content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.05 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

In the present invention, a mixing ratio of the maleic acid-modified polypropylene to the polypropylene depends upon the kind and the grade of selected maleic acid-modified polypropylene and polypropylene. If the mixing ratio of the maleic acid-modified polypropylene is too high, chemical resistances such as acid resistance and alkali resistance deteriorate, and if it is too low, adhesive force declines and the lining tends to peel. Hence, the proper mixing ratio is in the range of from 10/1 to 1/10.

With regard to the particle diameter of the powder suitable for the heated/melted lining, if it is less than 500 μm or less, a film having a uniform thickness can be obtained, but preferably, it is in the range of from 100 to 300 μm. Even if the powder having an average particle diameter of more than 500 μm is used, the lining film can be formed, but a long time is taken to melt the powder for the film formation, and the surface of the film becomes a rough state, which makes it impossible to form the lining film having a uniform thickness.

The lining material of the present invention is formed from the maleic acid-modified polypropylene and the polypropylene in a weight ratio of from 10/1 to 1/10. When the lining material is in a heated/melted state, both of these components are melted, and when the lining material is cooled/solidified, a carbonyl group contained in the maleic acid-modified polypropylene is strongly bonded to a metal, whereby the lining film having strong adhesive force can be obtained.

The heated/melted polypropylene lining material and the lining powder can be provided which can be applied to a high-temperature region to which a conventional polyethylene lining and rubber lining cannot be applied. In addition, this lining powder is excellent in chemical resistance and can also be applied to pipes having a complicate shape and different diameters.

Heretofore, in a device which is used in a high-temperature region and in severely corrosive circumstance to which the polyethylene lining and the rubber lining cannot be applied, a precious high-grade stainless steel or an expensive fluororesin-based lining material has been used. The polypropylene lining material and the lining powder of the present invention can be applied to such a device under the severe conditions, and the raw materials of the lining material and the powder are the fairly inexpensive maleic acid-modified polypropylene and polypropylene. Thus, the very inexpensive lining material and powder can be provided, and they have large values from industrial and economical viewpoints.

Furthermore, the present inventors have found that a good lining film which does not peel and which has strong adhesive force can be obtained by mixing a modified polypropylene and an ethylene propylene rubber with a polypropylene in a suitable ratio.

That is, the inventions are directed to:

(1) a polypropylene lining for a metal characterized by comprising a heated/melted mixture of 1 to 20 wt% of an ethylene propylene rubber and a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100,

(2) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber powder having an average particle diameter of 500 μm or less to a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 500 μm or less and a polypropylene powder in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=10/1 to 1/100, and then mixing them, and

(3) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, mixing them, melting and kneading the mixture, and then milling the same so that an average particle diameter may be 500 μm or less.

A metal which is useful in applying the present inventions is usually carbon steel. Furthermore, the content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.01 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

The content of the propylene in the ethylene propylene rubber which can be used in the present invention is usually in the range of from 10 to 40 wt%. In the present invention, a mixing ratio of the maleic acid-modified polypropylene to the polypropylene depends upon the kind and the grade of selected maleic acid-modified polypropylene and homopolypropylene, but the proper mixing ratio is in the range of from 10/1 to 1/100. The higher the mixing ratio of the maleic acid-modified polypropylene is, the stronger the adhesive force of the lining to the metal is. A ratio of the ethylene propylene rubber which is mixed with the above-mentioned mixture depends upon the kind and the grade of selected ethylene propylene rubber, but the proper mixing ratio is in the range of from 1 to 20 wt%. If the mixing ratio of the ethylene propylene rubber is too high, the strength of the lining declines.

Moreover, if the particle diameter of the powder suitable for the heated/melted lining is 500 μm or less, the film having a uniform thickness can be formed. Even if the powder having the average particle diameter more than 500 μm is used, the lining film can be formed, but a long time is taken to melt the powder for the film formation, and the surface of the film becomes a rough state, which makes it impossible to form the lining film having the uniform thickness.

The lining material of the present invention can be formed from the substance in which 1 to 20 wt% by outer percentage of the ethylene propylene rubber is added to the mixture of the maleic acid-modified polypropylene and the polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100. When the lining material is in a heated/melted state, both of the maleic acid-modified polypropylene and the polypropylene are melted, and when the lining material is cooled/solidified, a carbonyl group contained in the maleic acid-modified polypropylene is strongly bonded to a metal, whereby the lining film having strong adhesive force can be obtained.

On the other hand, when the lining material comprising the substance in which the ethylene propylene rubber is added to the mixture of the maleic acid-modified polypropylene and the polypropylene is in the heated and melted state, the maleic acid-modified polypropylene, the polypropylene and the ethylene propylene rubber are melted. The mixture of the maleic acid-modified polypropylene and the polypropylene (hereinafter referred to as "modified polypropylene" sometimes) is different from the ethylene propylene rubber in fluidity and solidification temperature, and therefore, when the lining material is cooled/solidified, the ethylene propylene rubber is solidified in a dispersed state in the modified polypropylene.

Since the creep stress of the ethylene propylene rubber is very small, the ethylene propylene rubber stretches at the time of solidification shrinkage and the stress is instantly relived to a level smaller than adhesive force. Hence, the lining which does not peel can be obtained.

In the lining powder for obtaining the lining of the present invention, the ethylene propylene rubber is added to the modified polypropylene, and therefore, owing to the stress relief function of the ethylene propylene rubber, the amount of the maleic acid-modified polypropylene can be reduced, as compared with a case where no ethylene propylene rubber is added.

The lining powder of the present invention can be melted and kneaded by the use of a melting kneader, and then milled so that an average particle diameter may be 500 μm or less to obtain a fine lining powder having the further improved adhesive properties. The mixing state of the thus obtained fine lining powder is more uniform than the mixture of the modified polypropylene, the ethylene propylene rubber having the small particle diameter.

Furthermore, the present inventors have found that when an antioxidant is added to an adhesive polypropylene prepared by mixing a polypropylene with a modified polypropylene and an ethylene propylene rubber, a tough film having good surface properties can be obtained without impairing adhesive properties and without deteriorating the resins.

That is, the inventions are directed to:

(1) a polypropylene lining for a metal characterized by comprising the steps of adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, heating/melting the resulting powder, and then cooling/solidifying the same to form a lining layer,

(2) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber powder having an average particle diameter of 500 μm or less to a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 500 μm or less and a polypropylene powder having an average particle diameter of 500 μm or less in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=10/1 to 1/100, further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, and then mixing them, and

(3) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, melting and kneading the resulting mixture, and then milling the same so that an average particle diameter may be 500 μm or less.

A metal which is useful in applying the present inventions is usually carbon steel.

The content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.01 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

The content of the propylene in the ethylene propylene rubber which can be used in the present invention is usually in the range of from 10 to 40 wt%. Moreover, in the present invention, the melt flow rate of the polypropylene is preferably between 0.1 and 40.

In the present invention, a mixing ratio of the maleic acid-modified polypropylene to the polypropylene depends upon the kind of selected maleic acid-modified polypropylene and polypropylene, but the proper mixing ratio is in the range of from 10/1 to 1/100. The higher the mixing ratio of the maleic acid-modified polypropylene is, the stronger the adhesive force of the lining to the metal is. A ratio of the ethylene propylene rubber which is mixed with the above-mentioned mixture depends upon the kind of selected ethylene propylene rubber, but the proper mixing ratio is in the range of from 1.0 to 20 wt%. If the mixing ratio of the ethylene propylene rubber is too high, the strength of the lining declines.

In the present invention, preferable examples of the antioxidant include high-molecular weight phenol-based antioxidants and phosphites (and phosphonites), and the amount of the antioxidant to be used depends upon the kind of antioxidant, but the proper amount is in the range of from 0.1 to 1.0 wt% by outer percentage based on the weight of the modified polypropylene.

A typical example of the antioxidant is a combination of a phosphorus-based working stabilizer such as tris(2,4-di-t-butylphenyl) phosphite and at least one selected from the group consisting of hindered phenol-based pentaerythrityltetrakis 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate!, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene and tris-(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate.

If the particle diameter of the powder is 500 μm or less suitable for the heated/melted lining, a film having a uniform thickness can be obtained. Even if the powder having the particle diameter larger than this size is used, the lining film can be formed, but a long time is taken to melt the powder for the film formation, and the surface of the film becomes a rough state, which makes it impossible to form the lining film having the uniform thickness.

The lining material of the present invention can be formed by adding 1.0 to 20 wt% of the ethylene propylene rubber to the mixture of the maleic acid-modified polypropylene and the polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, and further adding the antioxidant to the mixture. When the lining material is in a heated/melted state, both of the maleic acid-modified polypropylene and the polypropylene are melted, and when the lining material is cooled/solidified, a carboxyl group contained in the maleic acid-modified polypropylene is strongly bonded to a metal, whereby the lining film having strong adhesive force can be obtained.

On the other hand, when the lining material is in a heated/melted state, the ethylene propylene rubber added to the mixture of the maleic acid-modified polypropylene and the polypropylene is melted in the polypropylene, and when the lining material is cooled/solidified, the ethylene propylene rubber is dispersed in the polypropylene and the modified polypropylene, because the maleic acid-modified polypropylene, the polypropylene and the ethylene propylene rubber are different from each other in fluidity and solidification temperature. Since the creep deformation stress of the ethylene propylene rubber is very small, the ethylene propylene rubber stretches at the time of solidification shrinkage and the stress is instantly relived to a level smaller than adhesive force. Hence, the lining which does not peel can be obtained.

In addition, the antioxidant has a function for inhibiting the main chain of the polypropylene from thermally decomposing, and therefore the employment of the antioxidant permits thermal deterioration in the heating/melting step to decrease. In the lining powder of the present invention the antioxidant is added to the mixture of the polypropylene, the modified polypropylene and the ethylene propylene rubber, so that the thermal deterioration can be inhibited in the heating/melting step and so a tough film can be formed.

The lining powder of the present invention can be melted and kneaded by the use of a melting kneader, and then milled so that an average particle diameter may be 500 μm or less to obtain a fine lining powder having the further improved adhesive properties. The mixing state of the thus obtained fine lining powder is more uniform than the mixture of the modified polypropylene, the ethylene propylene rubber and the antioxidant having the small particle diameter.

Furthermore, the present inventors have intensively conducted experiments and investigations with the intention of developing a lining material which can be manufactured from a polypropylene sheet or a polypropylene stick having good adhesive properties, and as a result, they have found that when an antioxidant is added to an adhesive polypropylene prepared by mixing a polypropylene with a modified polypropylene and an ethylene propylene rubber, a tough film having good surface properties can be obtained without impairing adhesive properties and without deteriorating the resins.

That is, the inventions are directed to:

(1) a polypropylene lining sheet or stick comprising a resin composition obtained by adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, and further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, and

(2) a polypropylene lining method characterized by comprising the step of thermally fusing a sheet or a stick of a polypropylene resin composition of the above-mentioned (1) to a member to be lined.

The content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.01 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

The content of the propylene in the ethylene propylene rubber which can be used in the present invention is usually in the range of from 10 to 40 wt%. Moreover, in the present invention, the melt flow rate of the polypropylene is preferably between 0.1 and 40.

The sheet of the polypropylene resin composition according to the present invention is linable on the surface of a metal irrespective of its thickness, and when the sheet is thermally fused on the metal, a film having good adhesive properties can be formed. Furthermore, the stick of the polypropylene resin composition according to the present invention is linable on the surface of a metal irrespective of its thickness, and when the stick is arranged in one direction and thermally fused on the metal, a film having good adhesive properties can be formed. Thus, the stick is applicable for the repair of the lining.

The sheet or the stick of the polypropylene resin composition of the present invention can be fused on the surface of a metal to obtain a lining film having good adhesive properties to the metal.

Furthermore, the inventions are directed to:

(1) a lining method of a polypropylene characterized by comprising the steps of sprinkling the surface of a metal with a maleic acid-modified polypropylene powder, heating/melting the powder, sprinkling the melted maleic acid-modified polypropylene powder layer with a polypropylene powder, heating/melting the powder, and then cooling/solidifying the same to form a lining layer of the polypropylene, and

(2) a lining method of a polypropylene characterized by comprising the steps of sprinkling the surface of a metal with a maleic acid-modified polypropylene powder, heating/melting the powder, cooling/solidifying the same, sprinkling the resulting maleic acid-modified polypropylene film layer with a polypropylene powder, heating/melting the powder, and then cooling the same to form a lining layer of the polypropylene.

The content of maleic acid in the maleic acid-modified polypropylene which can be used in the inventions in claims 12 and 13 is 0.05% or more, preferably in the range of from 0.1 to 1.0%.

The maleic acid-modified polypropylene has a carbonyl group, and if heated/melted and then cooled/solidified, the maleic acid-modified polypropylene can be bonded to a metal. Thus, the present invention intends that the maleic acid-modified polypropylene film layer is first formed on the surface of the metal, and the polypropylene film is then formed on the maleic acid-modified polypropylene film layer.

The maleic acid-modified polypropylene has compatibility with the polypropylene, and so they can be bonded to each other by the heating/melting. Thus, the polypropylene lining of the present invention having the polypropylene film on its outer periphery can be obtained.

The film layer of the maleic acid-modified polypropylene interposed between the metal surface and the polypropylene film layer is preferably as thin as possible in order to improve the heat resistance of the polypropylene film. However, the minimum thickness of the film layer of the maleic acid-modified polypropylene depends upon the particle diameter of the maleic acid-modified polypropylene powder to be used for the film formation, and so the practical thickness of the film layer is in the range of from 50 to 200 μm.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of a matrix lined with a polypropylene.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt% and the pellets of a polypropylene having a melt flow index of 1.0 were separately milled by means of a refrigeration mill using liquid nitrogen, and each powder was classified by sieves into portions having average particle diameters of 1,000 μm, 500 μm, 300 μm and 100 μm. Afterward, the maleic acid-modified polypropylene powder was mixed with the polypropylene powder in blend ratios in Table 1 to obtain lining powders.

Furthermore, the pellets of the above-mentioned maleic acid-modified polypropylene was mixed with the pellets of the above-mentioned polypropylene in blend ratios shown in Table 1, and each mixture was then kneaded by means of a melting kneader, milled by the refrigeration mill, and then classified by the sieves to obtain lining powders having average particle diameters of 1,000 μm, 500 μm, 300 μm and 100 μm.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, heated on a heated plate so that its surface temperature might be 250°±20° C., sprinkled with the above-mentioned powder, and then heated/melted to obtain a lined steel plate having a film thickness of 1.5 mm. Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90° peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H₂ SO₄ and 100° were carried out. The results are set forth in Table 1.

                  TABLE 1                                                          ______________________________________                                                         Maleic Acid                                                                    Modification Ratio                                                                            Weight Ratio of                                                 of Maleic Acid-                                                                               Maleic Acid-Modified                                 Kind of    Modified Polypropylene                                                                        Polypropylene                                   No.  Lining Powder                                                                             (%)            to Polypropylene                                ______________________________________                                         1    Mixed powder                                                                              0.01           10/1                                            2    "          0.05           1/1                                             3    "          0.5            1/5                                             4    "          1.0             1/10                                           5    "          0.5            20/1                                            6    "          0.5             1/20                                           7    "          0.5            1/5                                             8    "          0.5            1/5                                             9    "          0.5            1/5                                             ______________________________________                                              Average Particle                                                                           Surface                                                            Diameter of State   Adhesive Force                                             Lining Powder                                                                              after   (kgf/    State after                                  No.  (μm)     Lining  10 mm width)                                                                            Durability Test                              ______________________________________                                          1   500         Good     4       Color changed                                                                  slightly                                      2   500         "        7       No problem                                    3   500         "        8       "                                             4   500         "        7       "                                             5   300         "       12       Color changed,                                                                 fine cracks occurred                          6   300         Peeled  --       --                                            7   300         Good    10       No problem                                    8   100         "       11       "                                             9   1000        Rough    6       "                                            ______________________________________                                                         Maleic Acid                                                                    Modification Ratio                                                                            Weight Ratio of                                                 of Maleic Acid-                                                                               Maleic Acid-Modified                                 Kind of    Modified Polypropylene                                                                        Polypropylene                                   No.  Lining Powder                                                                             (%)            to Polypropylene                                ______________________________________                                         10   Melted &   0.01           10/1                                                 kneaded                                                                   11   Melted &   0.05           1/1                                                  kneaded                                                                   12   Melted &   0.5            1/5                                                  kneaded                                                                   13   Melted &   1.0             1/10                                                kneaded                                                                   14   Melted &   0.5            20/1                                                 kneaded                                                                   15   Melted &   0.5             1/20                                                kneaded                                                                   16   Melted &   0.5            1/5                                                  kneaded                                                                   17   Melted &   0.5            1/5                                                  kneaded                                                                   18   Melted &   0.5            1/5                                                  kneaded                                                                   ______________________________________                                              Average Particle                                                                           Surface                                                            Diameter of State   Adhesive Force                                             Lining Powder                                                                              after   (kgf/    State after                                  No.  (μm)     Lining  10 mm width)                                                                            Durability Test                              ______________________________________                                         10   500         Good     5       Color changed                                                                  slightly                                     11   500         "        8       No problem                                   12   500         "       10       "                                            13   500         "        9       "                                            14   300         "       12       Color changed,                                                                 fine cracks occurred                         15   300         Peeled  --       --                                           16   300         Good    11       No problem                                   17   100         "       12       "                                            18   1000        Rough    7       "                                            ______________________________________                                    

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene having a melt flow index of 1.0 and the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40% were separately milled by means of a refrigeration mill using liquid nitrogen, and each powder was classified by sieves into portions having average particle diameters of 1,000 μm, 500 μm and 100 μm. Afterward, the resulting powders were mixed in blend ratios in Table 2 to obtain lining powders.

Furthermore, the pellets of the above-mentioned maleic acid-modified polypropylene, the above-mentioned polypropylene and the above-mentioned ethylene propylene rubber were mixed in blend ratios shown in Table 2, and each mixture was then kneaded by means of a melting kneader, milled by the refrigeration mill, and then classified by the sieves to obtain lining powders having average particle diameters of 1,000 μm, 500 μm and 100 μm.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, heated on a heated plate so that its surface temperature might be 250°±20° C., sprinkled with the above-mentioned powder, and then heated/melted to obtain a lined steel plate having a film thickness of 1.5 mm. Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90° peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H₂ SO₄ and 100° were carried out. The results are set forth in Table 2.

                  TABLE 2                                                          ______________________________________                                                                           Weight Ratio                                                 Maleic Acid                                                                               Propylene                                                                             of Maleic                                                    Modification                                                                              Content in                                                                            Acid-Modified                                                Ratio of Maleic                                                                           Ethylene                                                                              Polypropylene/                                               Acid-modified                                                                             Propylene                                                                             Ethylene                                          Kind of    Polypropylene                                                                             Rubber Propylene Rubber/                            No.  Lining Powder                                                                             (%)        (%)    Polypropylene                                ______________________________________                                          1   Mixed powder                                                                              0.01       25     1/0.4/1                                       2   "          0.05       25     1/0.4/1                                       3   "          0.10       25     1/0.4/1                                       4   "          0.50       25     1/0.4/1                                       5   "          1.00       25     1/0.4/1                                       6   "          0.50       10     1/0.4/1                                       7   "          0.50       40     1/0.4/1                                       8   "          0.50       25     1/0.2/1                                       9   "          0.50       25      1/0.025/1                                   10   "          0.50       25      10/1.2/1                                    11   "          0.50       25     5/0.7/1                                      12   "          0.50       25      1/0.21/1                                    ______________________________________                                              Average                                                                        Particle Diameter                                                              of Lining Powder                                                                           Surface State                                                                            Adhesive Force                                                                          State after                                No.  (μm)     after Lining                                                                             (kgf/10 mm φ)                                                                       Durability Test                            ______________________________________                                          1   100         Good       4       No problem                                  2   100         "         11       "                                           3   100         "         13       "                                           4   100         "         14       "                                           5   100         "         16       "                                           6   100         "         12       "                                           7   100         "         11       "                                           8   100         "         12       "                                           9   100         "         14       "                                          10   100         "         17       "                                          11   100         "         15       "                                          12   100         "         14       "                                          ______________________________________                                                                           Weight Ratio                                                 Maleic Acid                                                                               Propylene                                                                             of Maleic                                                    Modification                                                                              Content in                                                                            Acid-Modified                                                Ratio of Maleic                                                                           Ethylene                                                                              Polypropylene/                                               Acid-modified                                                                             Propylene                                                                             Ethylene                                          Kind of    Polypropylene                                                                             Rubber Propylene Rubber/                            No.  Lining Powder                                                                             (%)        (%)    Polypropylene                                ______________________________________                                         13   Mixed powder                                                                              0.50       25      1/1.2/10                                    14   "          0.50       25      1/5.7/50                                    15   "          0.50       25       1/12.2/100                                 16   "          0.05       25       1/23.3/200                                 17   "          0.50       25      1/1.2/10                                    18   "          0.50       25      1/1.2/10                                    19   Melted &   0.01       25     1/0.4/1                                           kneaded                                                                   20   Melted &   0.05       25     1/0.4/1                                           kneaded                                                                   21   Melted &   0.10       25     1/0.4/1                                           kneaded                                                                   22   Melted &   0.50       25     1/0.4/1                                           kneaded                                                                   23   Melted &   1.00       25     1/0.4/1                                           kneaded                                                                   24   Melted &   0.50       10     1/0.4/1                                           kneaded                                                                   ______________________________________                                              Average                                                                        Particle Diameter                                                              of Lining Powder                                                                           Surface State                                                                            Adhesive Force                                                                          State after                                No.  (μm)     after Lining                                                                             (kgf/10 mm φ)                                                                       Durability Test                            ______________________________________                                         13   100         Good      14       No problem                                 14   100         "         13       "                                          15   100         "         12       "                                          16   100         Peeled    --       --                                         17   500         Good      14       No problem                                 18   1000        Rough     11       "                                          19   100         Good       6       "                                          20   100         "         12       "                                          21   100         "         15       "                                          22   100         "         15       "                                          23   100         "         18       "                                          24   100         "         12       "                                          ______________________________________                                                                           Weight Ratio                                                 Maleic Acid                                                                               Propylene                                                                             of Maleic                                                    Modification                                                                              Content in                                                                            Acid-Modified                                                Ratio of Maleic                                                                           Ethylene                                                                              Polypropylene/                                               Acid-modified                                                                             Propylene                                                                             Ethylene                                          Kind of    Polypropylene                                                                             Rubber Propylene Rubber/                            No.  Lining Powder                                                                             (%)        (%)    Polypropylene                                ______________________________________                                         25   Melted &   0.50       40     1/0.4/1                                           kneaded                                                                   26   Melted &   0.50       25     1/0.2/1                                           kneaded                                                                   27   Melted &   0.50       25      1/0.025/1                                        kneaded                                                                   28   Melted &   0.50       25     10/1.2/1                                          kneaded                                                                   29   Melted &   0.50       25     5/0.7/1                                           kneaded                                                                   30   Melted &   0.50       25     1/0.2/1                                           kneaded                                                                   31   Melted &   0.50       25     1/1.2/10                                          kneaded                                                                   32   Melted &   0.50       25     1/5.7/50                                          kneaded                                                                   33   Melted &   0.50       25      1/12.2/100                                       kneaded                                                                   34   Melted &   0.50       25      1/23.3/200                                       kneaded                                                                   35   Melted &   0.50       25     1/1.2/10                                          kneaded                                                                   36   Melted &   0.50       25     1/1.2/10                                          kneaded                                                                   ______________________________________                                              Average                                                                        Particle Diameter                                                              of Lining Powder                                                                           Surface State                                                                            Adhesive Force                                                                          State after                                No.  (μm)     after Lining                                                                             (kgf/10 mm φ)                                                                       Durability Test                            ______________________________________                                         25   100         Good      12       No problem                                 26   100         "         13       "                                          27   100         "         15       "                                          28   100         "         19       "                                          29   100         "         17       "                                          30   100         "         15       "                                          31   100         "         15       "                                          32   100         "         14       "                                          33   100         "         12       "                                          34   100         Peeled    --       --                                         35   500         Good      16       No problem                                 36   1000        Rough     12       "                                          ______________________________________                                    

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene and the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40% were separately milled by means of a refrigeration mill using liquid nitrogen, and each powder was classified by sieves into portions having average particle diameters of 1,000 μm, 500 μm and 100 μm. Afterward, the resulting powders were mixed with a high-molecular weight phenol-based antioxidant and another antioxidant (IRGANOX B215, made by Ciba-Geigy) containing a phosphite (or a phosphonite) in blend ratios in Table 3 to obtain lining powders.

In this connection, the antioxidant (IRGANOX B215) used herein was a mixture of pentaerythrityltetrakis 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate! and tris(2,4-di-t-butylphenyl) phosphite in a ratio of 1:2.

Furthermore, the above-mentioned maleic acid-modified polypropylene, the polypropylene, the ethylene propylene rubber and the antioxidant were mixed in blend ratios shown in Table 4, and each mixture was then kneaded by means of a melting kneader, milled by the refrigeration mill, and then classified by the sieves into portions having average particle diameters of 1,000 μm, 500 μm and 100 μm to obtain lining powders.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, heated on a heated plate so that its surface temperature might be 250°±20° C., sprinkled with the above-mentioned powder, and then melted to obtain a lined steel plate having a film thickness of 1.5 mm.

Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90° peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H₂ SO₄ and 100° were carried out.

The results are set forth in Tables 3 and 4.

                                      TABLE 3                                      __________________________________________________________________________                Maleic Acid                                                                    Modification                                                                   Ratio of PP Content   Weight Ratio                                      Kind of                                                                               Modified PP*                                                                            in EPR**                                                                             Melt Flow                                                                             of Modified                                   No. Lining Powder                                                                         (%)      (%)   Rate of PP                                                                            PP/EPR/PP                                     __________________________________________________________________________      1  Mixed Powder                                                                          --       25    0.5     --/0.1/1.0                                    2  "      0.10     25    0.5    1.0/0.2/1.0                                    3  "      0.50     25    0.5    1.0/0.2/1.0                                    4  "      1.00     25    0.5    1.0/0.2/1.0                                    5  "      0.50     25    0.5    1.0/0.2/1.0                                    6  "      0.50     25    0.5    1.0/0.2/1.0                                    7  "      0.50     25    0.5    1.0/0.2/1.0                                    8  "      0.50     25    5.0    1.0/0.2/1.0                                    9  "      0.50     25    15.0   1.0/0.2/1.0                                   10  "      0.50     25    30.0   1.0/0.2/1.0                                   __________________________________________________________________________        Amount of                                                                      Antioxidant                                                                    to Mixture                                                                             Average                                                                of Modified                                                                            Particle Diameter                                                      PP, EPR and PP                                                                         of Lining Powder                                                                       Surface State                                                                        Adhesive Force                                                                          State after                                  No.                                                                               (%)     (μm) after Lining                                                                         (kgf/10 mm width)                                                                       Durability Test                              __________________________________________________________________________      1 0.25    100     Good  Peeled   --                                            2 0.25    100     "     12       No problem                                    3 0.25    100     "     14       "                                             4 0.25    100     "     13       "                                             5 --      100     Air bubbles                                                                          15       Air bubbles                                   6 0.50    100     Good  15       No problem                                    7 1.00    100     "     14       "                                             8 0.25    100     "     16       "                                             9 0.25    100     "     15       "                                            10 0.25    100     Partial liquid                                                                       15       "                                                               suspension                                                  __________________________________________________________________________                Maleic Acid                                                                    Modification                                                                   Ratio of PP Content   Weight Ratio                                      Kind of                                                                               Modified PP*                                                                            in EPR**                                                                             Melt Flow                                                                             of Modified                                   No. Lining Powder                                                                         (%)      (%)   Rate of PP                                                                            PP/EPR/PP                                     __________________________________________________________________________     11  Mixed powder                                                                          0.50     10    5.0    1.0/0.2/1.0                                   12  "      0.50     40    5.0    1.0/0.2/1.0                                   13  "      0.50     25    5.0    5.0/0.6/1.0                                   14  "      0.50     25    5.0     10.0/1.1/1.0                                 15  "      0.50     25    5.0     1.0/2.6/25.0                                 16  "      0.50     25    5.0      1.0/10.1/100.0                              17  "      0.50     25    5.0    1.0/0.4/1.0                                   18  "      0.50     --    5.0     1.0/--/1.0                                   19  "      0.50     25    5.0    1.0/0.2/1.0                                   20  "      0.50     25    5.0    1.0/0.2/1.0                                   __________________________________________________________________________        Amount of                                                                      Antioxidant                                                                    to Mixture                                                                             Average                                                                of Modified                                                                            Particle Diameter                                                      PP, EPR and PP                                                                         of Lining Powder                                                                       Surface State                                                                        Adhesive Force                                                                          State after                                  No.                                                                               (%)     (μm) after Lining                                                                         (kgf/10 mm width)                                                                       Durability Test                              __________________________________________________________________________     11 0.25    100     Good  14       No problem                                   12 0.25    100     "     16       "                                            13 0.25    100     "     18       "                                            14 0.25    100     "     17       "                                            15 0.25    100     "     15       "                                            16 0.25    100     "     14       "                                            17 0.25    100     "     15       "                                            18 0.25    100     "      2       Peeled                                       19 0.25    500     "     14       No problem                                   20 0.25    1000    Rough 15       Rough                                        __________________________________________________________________________      Note                                                                           *means a polypropylene, and                                                    **means an ethylene propylene rubber.                                    

                                      TABLE 4                                      __________________________________________________________________________                                       Amount of                                                                            Average                                             Maleic Acid                                                                           PP            Antioxidant                                                                          Particle                                            Modification                                                                          Content                                                                            Melt                                                                              Weight to Mixture of                                                                        Diameter                                                                            Surface                                                                              Adhesive                                 Ratio of                                                                              in  Flow                                                                              Ratio of                                                                              Modified PP,                                                                         of Lining                                                                           State Force State after               Kind of  Modified PP*                                                                          EPR**                                                                              Rate                                                                              Modified                                                                              EPR and PP                                                                           Powder                                                                              after (kgf/10                                                                              Durability            No. Lining Powder                                                                           (%)    (%) of PP                                                                             PP/EPR/PP                                                                             (%)   (μm)                                                                             Lining                                                                               width)                                                                               Test                  __________________________________________________________________________     21  Melted & kneaded                                                                        --     25  0.5                                                                               --/0.1/1.0                                                                            0.25  100  Good  Peeled                                                                               --                    22  "        0.10   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  100  "     14    No problem            23  "        0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  100  "     17    "                     24  "        1.00   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  100  "     16    "                     25  "        0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           --    100  Rough 16    "                     26  "        0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           0.50  100  Good  17    "                     27  "        0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           1.00  100  "     17    "                     28  "        0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  100  "     18    "                     29  "        0.50   25  15.0                                                                              1.0/0.2/1.0                                                                           0.25  100  "     17    "                     30  "        0.50   25  30.0                                                                              1.0/0.2/1.0                                                                           0.25  100  Partial liquid                                                                       18    "                                                                  suspension                        31  Melted & kneaded                                                                        0.50   10  5.0                                                                               1.0/0.2/1.0                                                                           0.25  100  Good  17    No problem            32  "        0.50   40  5.0                                                                               1.0/0.2/1.0                                                                           0.25  100  "     18    "                     33  "        0.50   25  5.0                                                                               5.0/0.6/1.0                                                                           0.25  100  "     20    "                     34  "        0.50   25  5.0                                                                               10.0/1.1/1.0                                                                          0.25  100  "     18    "                     35  "        0.50   25  5.0                                                                               1.0/2.6/25.0                                                                          0.25  100  "     17    "                     36  "        0.50   25  5.0                                                                               1.0/10.1/100.0                                                                        0.25  100  "     17    "                     37  "        0.50   25  5.0                                                                               1.0/0.4/1.0                                                                           0.25  100  "     18    "                     38  "        0.50   --  5.0                                                                               1.0/--/1.0                                                                            0.25  100  "      3    Peeled                39  "        0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  100  "     17    No problem            40  "        0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  100  "     17    "                     __________________________________________________________________________      Note:                                                                          * means a polypropylene, and ** means an ethylene propylene rubber.      

A preferable embodiment of the inventions will be described.

EXAMPLE 1

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene, the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40%, a high-molecular weight phenol-based antioxidant and another antioxidant (IRGANOX B215, made by Ciba-Geigy) containing a phosphite (or a phosphonite) were mixed with in blend ratios shown in Tables 5 and 6, and each mixture was then kneaded by means of a melting kneader, and then molded by the use of a sheet molding machine to obtain sheets having film thicknesses of 200 μm, 500 μm, 1,000 μm and 2,000 μm.

In this connection, the antioxidant (IRGANOX B215) used herein was a mixture of pentaerythrityltetrakis 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate! and tris(2,4-di-t-butylphenyl) phosphite in a ratio of 1:2.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was decreased, shot-blasted, and then heated on a heated plate so that its surface temperature might be 250°±20° C., and the above-mentioned sheet was pressed against the heated steel plate to fuse it thereon, thereby obtaining the steel plate lined with the polypropylene sheet.

Afterward, the observation of the surface state of each sheet lining, the measurement of adhesive force by a 90° peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H₂ SO₄ and 100° were carried out.

The results are set forth in Table 5.

                                      TABLE 5                                      __________________________________________________________________________                             Amount of                                                 Maleic Acid                                                                           PP            Antioxidant                                                                          Thickness                                           Modification                                                                          Content                                                                            Melt      to Mixture of                                                                        of Sheet                                                                             Surface  Adhesive                             Ratio of                                                                              in  Flow                                                                              Modified                                                                              Modified PP,                                                                         of PP Resin                                                                          State    Force State after                    Modified PP*                                                                          EPR**                                                                              Rate                                                                              PP/EPR/PP                                                                             EPR and PP                                                                           Composition                                                                          of       (kgf/10 mm                                                                           Durability                  No.                                                                               (%)    (%) of PP                                                                             (weight ratio)                                                                        (%)   (μm)                                                                              Lining   width)                                                                               Test                        __________________________________________________________________________     1  --     25  0.5                                                                               --/0.1/1.0                                                                            0.25  500   Metal surface                                                                           Peeled                                                                               --                                                              peeled                                     2  0.10   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  500   Uniform and good                                                                        14    No problem                  3  0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  500   "        12    "                           4  1.00   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  500   "        13    "                           5  0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           --    500   Air bubbles                                                                             13    Air bubbles                 6  0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           0.50  500   Uniform and good                                                                        14    No problem                  7  0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           1.00  500   "        14    "                           8  0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  500   "        15    "                           9  0.50   25  15.0                                                                              1.0/0.2/1.0                                                                           0.25  500   "        16    "                           10 0.50   25  30.0                                                                              1.0/0.2/1.0                                                                           0.25  500   "        16    "                           11 0.50   10  5.0                                                                               1.0/0.2/1.0                                                                           0.25  500   Uniform and good                                                                        15    No problem                  12 0.50   40  5.0                                                                               1.0/0.2/1.0                                                                           0.25  500   "        15    "                           13 0.50   25  5.0                                                                               5.0/0.6/1.0                                                                           0.25  500   "        16    "                           14 0.50   25  5.0                                                                               10.0/1.1/1.0                                                                          0.25  500   "        16    "                           15 0.50   25  5.0                                                                               1.0/2.6/25.0                                                                          0.25  500   "        17    "                           16 0.50   25  5.0                                                                               1.0/10.1/100.0                                                                        0.25  500   "        14    "                           17 0.50   25  5.0                                                                               1.0/0.4/1.0                                                                           0.25  500   "        14    "                           18 0.50   --  5.0                                                                               1.0/--/1.0                                                                            0.25  500   Partially peeled                                                                         2    Peeled                      19 0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  200   Uniform and good                                                                        15    No problem                  20 0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  1000  "        16    "                           21 0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  2000  "        15    "                           __________________________________________________________________________      Note:                                                                          * means a polypropylene, and ** means an ethylene propylene rubber.      

EXAMPLE 2

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene, the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40%, a high-molecular weight phenol-based antioxidant and another antioxidant (IRGANOX B215) containing a phosphite (or a phosphonite) were mixed with in blend ratios shown in Table 6. Each mixture was then kneaded by means of a melting kneader, and then extruded through a cylinder head to obtain strands, and sticks were prepared from the strand.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, and then heated on a heated plate so that its surface temperature might be 250°±20° C., and the above-mentioned stick was pressed against the heated steel plate to fuse it thereon, thereby obtaining the steel plate lined with the polypropylene.

Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90° peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H₂ SO₄ and 100° were carried out.

The results are set forth in Table 6.

                                      TABLE 6                                      __________________________________________________________________________                             Amount of                                                 Maleic Acid                                                                           PP            Antioxidant                                                                          Size of                                             Modification                                                                          Content                                                                            Melt      to Mixture of                                                                        Stick of                                                                             Surface                                                                            Adhesive                                  Ratio of                                                                              in  Flow                                                                              Modified                                                                              Modified PP,                                                                         PP Resin                                                                             State                                                                              Force State after                         Modified PP*                                                                          EPR**                                                                              Rate                                                                              PP/EPR/PP                                                                             EPR and PP                                                                           Composition                                                                          of  (kgf/10 mm                                                                           Durability                       No.                                                                               (%)    (%) of PP                                                                             (weight ratio)                                                                        (%)   (mm)  Lining                                                                             width)                                                                               Test                             __________________________________________________________________________     22 0.50   25  0.5                                                                               1.0/0.2/1.0                                                                           0.25  3     Good                                                                               15    No problem                       23 0.50   25  5.0                                                                               1.0/0.2/1.0                                                                           0.25  3     "   14    "                                24 0.50   25  15.0                                                                              1.0/0.2/1.0                                                                           0.25  3     "   16    "                                25 0.50   25  30.0                                                                              1.0/0.2/1.0                                                                           0.25  3     "   14    "                                __________________________________________________________________________      * means a polypropylene, and ** means an ethylene propylene rubber.      

A preferable embodiment of the inventions will be described in reference to FIG. 1.

In FIG. 1, reference numeral 1 is a metal matrix (carbon steel) having a diameter of 120 mm and a thickness of 5 mm in which the surface of the metal matrix is cleaned by degreasing, washing and blasting, numeral 2 is a maleic acid-modified polypropylene film formed by heating/melting (250°±20° C.) a maleic acid-modified powder having a melt index of 10 and an average diameter of 100 μm on the surface of the metal matrix 1. Numeral 3 is a polypropylene film formed by heating/melting (250°±20° C.) a polypropylene powder having a melt index of 5 and an average diameter of 100 μm on the maleic acid-modified polypropylene film 2. The formation of the polypropylene film 3 can be carried out at a time when the previously formed maleic acid-modified polypropylene film 2 is still in a melted state or in a cooled/solidified state. In Table 7, there are shown the results of the adhesive force and the durability test (for 6 months under conditions of 5% sulfuric acid and 100° C.) of the polypropylene lining materials which lining has been made by two methods of the present invention.

                                      TABLE 7                                      __________________________________________________________________________        Thickness of                                                                          Thickness of                                                                          Thickness of                                                                          Thickness of                                              Film of 0.05%                                                                         Film of 0.5%                                                                          Film of 1.0%                                                                          Film of 0.01%                                                                         Thickness                                          Maleic Acid-                                                                          Maleic Acid-                                                                          Maleic Acid-                                                                          Maleic Acid-                                                                          of Poly-                                           Modified                                                                              Modified                                                                              Modified                                                                              Modified                                                                              propylene      Adhesive Force                      Polypropylene                                                                         Polypropylene                                                                         Polypropylene                                                                         Polypropylene                                                                         Film  Surface State                                                                           after Lining                                                                            State after             No.                                                                               (μm)                                                                               (μm)                                                                               (μm)                                                                               (μm)                                                                               (μm)                                                                              after Lining                                                                            (kgf/10 mm                                                                              Durability              __________________________________________________________________________                                                            Test                    1  None   None   None   None   2000  Peeled from                                                                             --       --                                                           metal surface                             2  None   None   None   None    200  Peeled from                                                                             --       --                                                           metal surface                             3  200    None   None   None   1000  Uniform & good                                                                           7       No problem              4  None    50    None   None   2000  "        12       "                       5  None    200   None   None   2000  "        11       "                       6  None    500   None   None   2000  "        10       "                       7  None   1000   None   None   1000  "        10       Wavy wrinkles                                                                  occured slightly        8  None   2000   None   None    500  Wrinkles occured                                                                        10       Wavy wrinkles                                                slightly on poly- occured slightly                                             propylene film layer                      9  None   None   200    None   1000  Uniform & good                                                                          12       No problem              10 None   None   None   200    1000  "         2       Peeled                  __________________________________________________________________________ 

We claim:
 1. A method for preventing corrosion of a metal piece under severe operating conditions comprising the steps of obtaining a metal piece whose surface will come into contact with a corrosive fluid of an exhaust gas desulfurization device, a gas purification plant, a seawater-fresh water converter or a thermal power plant, sprinkling the surface of the metal piece with a polypropylene lining powder which comprises a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 100 to 500 μm and having a maleic acid modification ratio of from 0.05 to 1.0 wt% and a polypropylene powder having an average particle diameter of 100 to 500 μm in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=1/1 to 1/10, heating/melting the polypropylene lining powder, and then cooling/solidifying the same to form a lining layer of the polypropylene.
 2. A method of claim 1 including the step of placing the surface of the metal piece into contact with the corrosive fluid at a temperature of about 100° C.
 3. A method for preventing corrosion of a metal piece under severe operating conditions comprising the steps of obtaining a metal piece whose surface will come into contact with a corrosive fluid of an exhaust gas desulfurization device, a gas purification plant, a seawater-fresh water converter or a thermal power plant, sprinkling the surface of the metal piece with a polypropylene lining powder having an average particle diameter of 100 to 500 μm which comprises a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.05 to 1.0 wt% and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=1/1 to 1/10, heating/melting the polypropylene lining powder, and then cooling/solidifying the same to form a lining layer of the polypropylene.
 4. A method of claim 3 including the step of placing the surface of the metal piece into contact with the corrosive fluid at a temperature of about 100° C. 